RF power amplifier packages may comprise a package and a semiconductor die, and the die may be arranged inside the package and provided with an RF power transistor. The RF power transistor may have an output capacitance and may be configured to amplify signals at an operational frequency. An impedance network may be arranged inside the package for providing impedance matching and/or filtering. These packages may be used in base stations for the mobile communications market.
The evolution of the mobile communications market is essentially driven by a continuously increasing amount of data and transmission speed, resulting in a need for an increasingly larger instantaneous signal bandwidth. To linearly amplify wide band signals and minimize the memory effects, it may be useful to terminate the second order intermodulation distortion (IMD) products occurring at relatively low frequencies with very low impedances. If the impedance seen by an RF power transistor at these frequencies is too high, the amplitude of the undesired signal components may increase, and the biasing of the transistor may be affected by feedback of the low-frequency IMD products into the biasing circuitry.
Another aspect of RF power amplifiers is the ability to deliver the output signal at the desired power level. To that end, it may be useful for the RF power transistor to be appropriately impedance matched. In particular, the effective load seen by the RF power transistor should only have a small reactive part. In practice however, most RF power transistors may have a considerable output capacitance. As an example, a laterally diffused metal-oxide-semiconductor (LDMOS) transistor, configured to provide output powers in the range of 150 Watts, may have a drain-source capacitance of approximately 50 pF. In the frequency band of interest, which in the mobile base station market ranges from 600 MHz to 3.5 GHz and more, this capacitance strongly influences the impedance seen at the drain of the LDMOS.